This study of the interrelationships of body composition, nutrition and physical activity is a longitudinal study of aging. It provides a description of these characteristics for participants in the Baltimore Longitudinal Study. It provides opportunity to relate changes in these basic characteristics of the individual participants to changes in other biochemical, physiological and psychological measurements. A variety of non-invasive techniques are employed. They include the Behnke Anthropometric Index, skinfold thickness measurements, height, weight, twenty-four hour creatinine excretion, total body potassium determination, basal metabolism determinations, Garn X-ray fat thickness measurements, a diet diary, and an activity questionnaire. Previously, measures of total body density and total body water have been made in longitudinal studies participants. Body density corrected for differences in body water content have been compared with the Behnke Index and other conventional anthropometric indices (such as ponderal index).